how do I use the array first element to do some math in python-CodePudding

I have an

array([[5.1, 3.5, 1.4, 0.2],
       [4.9, 3. , 1.4, 0.2],
       [4.7, 3.2, 1.3, 0.2],
       [4.6, 3.1, 1.5, 0.2])

I want to get the sum of the equation:

(5.1 - 1)

(4.9 - 1)

(4.7 - 1)

(4.6 - 1)

How do I get the every arrays' first element?

CodePudding user response：

Assuming this is a Numpy array, you can just subtract from the first column and let broadcasting do the work. If you want the sum of that result, just use sum():

import numpy as np

arr = np.array([
    [5.1, 3.5, 1.4, 0.2],
    [4.9, 3. , 1.4, 0.2],
    [4.7, 3.2, 1.3, 0.2],
    [4.6, 3.1, 1.5, 0.2]
])

a = arr[:, 0] - 1
#array([4.1, 3.9, 3.7, 3.6])

a.sum()
15.299999999999999

If you are bothered by the inexact sum, make sure you read Is floating point math broken?

CodePudding user response：

There's an axis argument in array.sum that you can set to sum the array vertically.

(arr-1).sum(axis=0)
array([15.3,  8.8,  1.6, -3.2])